When it is desired to determine the approximate position of an object equipped with a responder, for example an aircraft, starting from a receiver, for example a ship equipped with a transponder, a known technique is to interrogate the responder and to analyze the time needed for the reception of the response generated by the latter. This is because the angular sector within which the responder is located and the distance at which it is traveling can be determined by ascertaining the antenna coverage of the receiver and of the time taken between the transmission of the interrogation and the reception of the response transmitted by the responder, respectively.
However, the precision obtained by this method is sometimes insufficient. In order to determine the position of an object more precisely, specific pieces of equipment may be installed, both in the interrogation system and in the object equipped with the responder, in such a manner as to allow the object to transmit its precise coordinates by radio communications and in a format permitting a high enough precision. However, this solution requires the use of additional hardware.
It is also possible to forego the conventional use of the identification codes transmitted by a responder in order to transmit, in place of such a code, a coordinate formatted in the manner of an identification code. Nevertheless, some identification formats are very limited by the size of the messages to be transmitted. By way of example, for responders communicating via the SIF (Selective Identification Features) protocol, only 13 bits are available for transmission of a data value, whereas, for example, in order to obtain a precision of around 330 m, 16 bits are required for transmission of a latitude coordinate and 17 bits are required for transmission of a longitude coordinate, since the Earth's circumference is approximately equal to 40,000 km (40,000/217<330).
An algorithm known by the name “Compact Position Reporting” or CPR allows the number of bits needed for the transmission of the coordinates of an object to be reduced. This algorithm is detailed in the following document: “SSR Improvements and collision avoidance systems panel working group 1, an algorithm for compact position reporting (CPR)”, R. D Grappel, V. A. Orlando, SICASP/WG-1, WP/1-403, Apr. 26, 1994.
As is illustrated by FIG. 1a, a first subdivision is performed by the CPR algorithm to divide the Earth's sphere 101 into parallel strips 150 at the equator 116, the strips having the same height H. This first subdivision allows the number of bits needed for coding the latitude to be reduced, and the latitude is referenced locally with respect to one of said strips 150. Knowing the region in which the receiver is located and the range limit of this receiver means that the latitude of the transmitter can readily be decoded using the receiver. Thus, to take the aforementioned example, if, using the CPR algorithm, the northern hemisphere is divided into 15 strips of 6° of latitude each, a precision of around 330 m may be obtained by coding the latitude over 10 bits instead of 16 bits without subdivision, a strip of latitude being approximately 670 km from north to south (670 km/210<330 m).
Furthermore, as illustrated in FIG. 1b, a second subdivision by the CPR algorithm is carried out in order to code the longitude over a reduced number of bits. This second subdivision divides the Earth's sphere 101 into separate cells bounded on the east and on the west by meridians 102a, 102b and on the north and on the south by parallels 104a, 104b, the cells being included within latitude sections 106a, 106b, 106c, 106d, 106e. The longitude of the object 108 is then referenced relative to the cell 110 in which the object is located. Thus, taking the aforementioned example, using the CPR algorithm to create cells of 360 nautical miles in width, a precision of around 330 m can be obtained by coding the longitude over 11 bits instead of 17 bits (360×1852/211<330 m, 1 nautical mile being equivalent to 1852 m). Furthermore, according to this second CPR subdivision, the width L of the northern border—for the northern hemisphere or the southern border for the southern hemisphere—of a cell is invariant, in such a manner that by changing latitude section, the angular widths of the cells vary. In other words, the number of cells present at a given latitude decreases with distance away from the equator 116 and the cells are shifted with respect to one another from one latitude section 106a to another 106b. Thus, owing to these shifts 118 between cells of different latitudes, the longitude cannot be decoded without knowing the latitude of the object.
A first solution consists in transmitting the longitude and the latitude in one and the same message, in such a manner that the longitude can always be decoded. However, the mode of transmission used does not always allow a total of 21 bits to be transmitted in a message (10 bits+11 bits), as illustrated hereinabove with the example of a responder transmitting messages with a size of 13 bits.
A second solution consists in transmitting the latitude in a first message, the longitude in a second message, then decoding the longitude by virtue of the information on latitude previously received. However, in view of the frequent failures in the transmissions, the probability of obtaining two successful transmissions successively is low. This solution is not therefore satisfactory.